1. Field
The disclosure relates to the design of audio amplifiers, and, in particular, to waveform shaping for audio amplifiers.
2. Background
In the operation of audio power amplifiers (PA's), certain operational scenarios call for the output of the audio PA to be brought from one DC voltage level to another level. For example, during initial start-up or power-down of the audio PA, a DC bias or offset voltage may be established at or removed from the PA output. In particular, a DC level may normally be present at the output of the amplifier, e.g., during class A or AB operation. In this case, Vout may be brought up from 0 Volts up to the DC level during start-up of the amplifier, and conversely, Vout may be brought down from the DC level to 0 Volts during shutdown of the amplifier. Alternatively, the PA output voltage may also transition between two or more voltage levels during an impedance measurement mode, e.g., wherein a plurality of voltage-current measurements are made at the output of the PA to determine the impedance of the audio load.
In either scenario, the transitioning up or down of the amplifier output from one DC level to another DC level may undesirably generate certain audio artifacts. For example, such transitions may cause audible “pops” or “clicks” that are perceptible by a user of the audio device. To minimize such audio artifacts introduced when the amplifier output transitions from one DC level to another, a “ramping” waveform may be adopted. In particular, instead of allowing the amplifier to transition directly from one DC level to another in an uncontrolled manner, the transition may be controlled, e.g., linearly spread out over a period of time. Through such “ramping,” audio artifacts at the amplifier output may be noticeably reduced. However, it would be desirable to provide techniques to further reduce such audio artifacts, as well as provide for faster convergence to the final desired DC level over time.
In general, linear ramping profiles may not necessarily offer the most optimal tradeoff between minimizing the audio artifacts generated by the transition, and minimizing the required transition time. It would be desirable to provide additional techniques for minimizing audio artifacts which simultaneously offer rapid convergence of the amplifier output to the desired DC voltage level.